Within an overview article recently published in WIREs Water, Christopher J. Duffy attempts to trace the historical theme of the terrestrial hydrologic cycle as it was understood and represented by scholars of natural philosophy, from the ancient Greeks to the end of the 18th century. Using historical images and text, his paper follows a path of how we may have arrived at the modern interpretation; and the evidence proposed for sources, origins and early attempts at quantification. By comparing models of the hydrologic cycle from text and images one can begin to construct a plausible story for its evolution and the contributions of important actors. Particular attention is paid to the origin of springs, rivers, and groundwater and their relation to the sense of balance that emerges.

The notion of balance, as introduced in Joel Kaye’s A History of Balance, 1250-1375, captures a central theme for how scholars of natural history, medieval mathematicians, designers of scientific instruments and early waterworks engineers seemed to have viewed the complexities and evaluated the evidence for how water moves in the terrestrial landscape.

A fundamental question that emerged early in this story was whether rainfall was sufficient to cause springs and rivers. Consider an exemplar from Athanasius Kircher’s Mundus Subterraneous, 1641. Kircher proposes that the ultimate source for springs and rivers was from the sea, where water from the seabed passes through the subsurface abyss, ascends into cold mountain cavities (by various mechanisms), and then returns to the sea as rivers. Yi-Fu Tuan refers to this as the “reverse hydrologic cycle”. In western literature the reverse hydrologic cycle was a leading model, with arguments for and against the notion given by Aristotle (323 BCE), the Romans Pliny (c. 79) and Seneca (c. 65), Isodore of Seville (c. 615), Leonardo DaVinci (1502), Bernard Palissy (1580), Pierre Perrault (1674), Edmund Halley (1692), John Dalton (1799), and many others.

Proponents of the “reverse hydrologic cycle” did not seem to preclude a pluvial influence on springs and rivers, rather the precipitation theory, or rain-causes-rivers, was held to be insufficient as a source. So in many ways their model of balance was always proportional or conditional solutions to the problem, each with a greater or lessor degree of certainty. The quote from Seneca makes the case:

“…as a diligent digger of vines, I can affirm from observation that no rain is ever so heavy as to wet the ground to a depth of more than 10 feet … How then, can rain, …store up a supply sufficient for rivers?”

There were other traditions unencumbered by the reverse hydrologic cycle. An important case in point was an 11th century text by Mohammed ibn al-Karaji, a Persian mathematician who made important contributions to algebra and invented instruments for surveying underground tunnels for accessing water in the desert basins. Al-Karaji wrote The Extraction of Hidden Waters, a treatise on the design and operation of qanats and the hydrology of desert basins in Persia. Qanat technology was important in the history of human settlement and irrigation in desert oases along the Silk Road, to Mesopotamia, across North Africa to present day Spain. Al-Karaji gives a strikingly modern view of the source of groundwater and springs, and his descriptions and deep understanding of the hydrologic cycle were unsurpassed until the 18th or 19th century.

It is also interesting to note that starting from the reverse model of balance, or we might say the “wrong” model for the origin of rivers, Leonardo DaVinci in 1502 made important progress on local conservation laws as applied to flow in river networks, which were later formalized by Galileo’s disciple Castelli (1660).

By the end of the 18th and into the early 19th century, the modern model of the terrestrial hydrologic cycle that emerged served as an intellectual foundation for the mathematical Earth science themes that followed. It was the physicist-philosopher Ernst Mach (1911) who claimed that something came before mechanical laws that had deeper roots in earlier natural history. It was necessary, it seems, to devise a sense of balance on which we might compare and comprehend competing views of the physical world. This theme was shared by the ancient scholars of natural history included here, and serves as an important example of early steps in understanding the laws of nature.

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